Lecture 1 - Introduction

Question 1

What are the 2 steps of the HTTPS protocol?

Answer 1

1. Key exchange: Generation of a shared key using asymmetric cryptography
2. Secure communication: Secure communication with the help of the shared key using symmetric cryptography

Question 2

Explain secure multi-party computation

Answer 2

In the center there is a trustworthy party, other parties send their xi (let’s say for example there are 3), the trustworthy party calculates the y=f(x1,x2,x3) and sends it to all parties, this key will be used to encrypt messages.

Question 3

What’s the secure multi-party theorem

Answer 3

Each trusted party can be realized by a protocol.

Question 4

What’s the basic idea of an encryption scheme?

Answer 4

both actual parties have a secret key (k), sending party encrypts the message using the secret key and transfers the ciphertext, if the ciphertext is taken, without the key it cannot be deciphered, the receiver gets the ciphertext, deciphers it using the secret key k, and reads the message

Question 5

Explain the syntax of a symmetric cipher

Answer 5

An encryption scheme is a tuple (Gen, Enc, Dec) where
Gen: Probabilistic key generation algo, which takes as input 1^n and returns key k.
Enc: Encryption algo that takes input k and message m ∈ M and outputs 𝐜 ∈ C. We write c <– Enck(m)
Dec: Decryption algo gets as input the key k and ciphertext c and outputs the message m. Write m = Deck(c)

Spaces:
K - Keyspace
M - Plaintext space
C - Ciphertext space

Correctness: for each k and m holds: Deck(Enck(m)) = m

Question 6

What is kerckhoffs principle?

Answer 6

The cipher should remain secure, even if the attacker knows the cipher algo (i.e. attacker knows (Gen, Enc, Dec).

The only thing that is secret is a short key k, which is chosen at random.

Question 7

Why kerchkhoffs principle?

Answer 7

1. In commercially used products, it is difficult to keep the specification secret (reverse engineering).
2. Short keys are easier to protect, to create and to exchange
3. The security of the design can be analyzed publicly.

Question 8

What is the shift cipher

Answer 8

Shifting each letter by k places in the alphabet

Question 9

How can we break substitution ciphers?

Answer 9

Substitution ciphers have a fixed mapping for each letter to another letter/symbol, but a statistical approach may be used.

If we know the most used letter in the eng alphabet and we know the message is in english, we can compare and decipher that way.

Question 10

What are the 3 principles of modern cryptography?

Answer 10

1. Formal definition - security must be precisely defined (sec goals, comparison between schemes, security proofs). Has 2 components (Goals of the adversary, adversary model)
2. Construction (of a crypto scheme) - Construction of complex crypto scheme from simple crypto primitives, sometimes based on number-theoretical assumptions, design of base primitives should always be left to experts.
3. Sec Analysis - Proof of security of the construction: carrying out a reduction

• First try: Test security against all attacks
• Not possible: There are too many attacks!
• Second try: Security based on an assumption
• Reduction proof: The only way to break the cryptoscheme is to break the assumption